Лако-красочные материалы — производство

Alan L. Lambuth ‘

Boise Cascade Corporation, Boise, Idaho, U. S.A.

I. INTRODUCTION

Human beings have apparently had a propensity for gluing things together since the dawn of recorded history and probably before. By experimentation, we learned over time that certain natural products or extractives could develop bonds in wood of sufficient strength to break in the material being glued rather than in the glue film itself. These observations ultimately led to papyrus laminating, decorative wood veneering, and furniture and musi­cal instrument assembly, practiced by the early Chinese and Egyptians and virtually every other civilization since that time using nature’s own glues [l-3].

By today’s standards, the ancient adhesive raw material choices were limited. Starch, blood, and collagen extracts from animal bones or hides were the principal early sources. Somewhat later, milk protein and fish skin extracts were discovered and included. Interestingly, vegetable proteins appear not to have been utilized as adhesives until recent times [4]. Tree pitch and petroleum bitumen were known and exploited as weath­erproof coatings and caulks but not as adhesives, due to their plastic-flow behavior [5]; that is, the joints would creep and thus could not be used for structural support. Adhesive durability has always been a problem. Although starch and protein glues were able to maintain long-term adhesive strength when kept completely dry, none were to any significant degree resistant to water or mold. Heat-cured blood glues and casein glues eventually provided some moisture resistance, but that was as durable as the technology of the times permitted. This fact limited the uses of adhesives strictly to interior or at least covered exterior applications from early historical times down through the Industrial Revolution and nearly to the present. From the middle of the nineteenth century onward, knowledge regarding more efficient means of dispersing and denaturing protein adhesives began to accumulate [6,7]. The results were significant improvements in protein glue working properties, bond strength, and water resistance, but still well short of true exterior durability. The urgencies of World War I brought protein-based adhesives to their nearest attainment of this goal in the form of chemically denatured, heat-set blood glues. These were used to assemble, among other things, laminated wooden airplane propellers and structural components in support of the war effort [8].

In the early 1920s the first of the phenol-formaldehyde and urea-formaldehyde resin adhesives were developed [9,10]. These early resins were slow to cure and somewhat difficulDeceased.

to use, but they ushered in the era of “thermoset’’ polymeric adhesives and true exterior durability. The urgencies of World War II forced the rapid development of these and other synthetic resin glues for their water and weather resistance but left protein adhesive tech­nology to evolve and flourish in applications for which ‘‘interior’’ durability was still ade­quate [11]. In the years following the war, the greatly expanded and partly idle petrochemical industry went looking for appropriate new markets. Among the opportu­nities identified were synthetic resin adhesive applications. Within a decade, these were converted from costly wartime adhesive specialties to bulk commodity glues. Throughout this synthetic resin expansion, however, protein adhesives held onto their ‘‘interior durable’’ product applications, due in large part to their unique combination of low cost and cold­curing capability or, alternatively, very fast hot press curing (about twice as fast as pheno — lics) [12,13]. This situation continued into the 1960s, at which time the price of petrochemi­cally based adhesives had become so low that they literally displaced protein adhesives from their traditional interior markets. Specifically, phenolic and urea-formaldehyde resins replaced blood, soybean, and starch glues in all plywood and composite wood panels; resorcinol-formaldehyde resins replaced casein glues in lumber laminating and millwork applications; and poly(vinyl acetate) and acrylic emulsion glues replaced virtually all col­lagen adhesives (animal and fish bone/skin derived) from furniture, musical instruments, and general interior wood assembly [14]. Limited and specialized applications for protein glues, mainly in combination with synthetic resin polymers, continue to the present, how­ever. These are discussed in the following sections. It is worth noting that protein glue technology in each of the application areas described above remains fully useful and indus­try approved in a backup sense and could be reintroduced rapidly if world petroleum resources were to be threatened again through events beyond national control, as in 1973.

In this chapter we address three of the most widely used families of protein-based adhesives for wood: soybean, blood, and casein. The technology presented is drawn pri­marily from the years 1930 through about 1960, when the consumption and technical refinements of these adhesives were at their peak. Soybean glues are discussed first because they were often utilized in combination with blood or casein to yield adhesives of inter­mediate performance properties as well as being used alone.